Brian: Lake Mead, the country's largest reservoir. The Hoover Dam blocks the Colorado River and forms Lake Mead. It's all a wonder of engineering - but thing don't look so wonderful right now. That bathtub ring means one thing : water levels are dropping. So, the depletion of the water supply that comes from the Colorado River is pretty evident. The white line, the demarcation you see on the mountains here, is really an alkaline marker, or a line that shows you where the water once was - and that water line drops down more than 100 feet and has done so since the 1990's. So where is all the water going? Well, the Colorado River is used by seven states. It's the nations driest region so each state has staked its claim to the water. Now, the growth of western cities is sucking the river dry.